In recent years, the search for new reserves of fuel, particularly new deposits of oil and gas, has been extended to the ocean floor. As exploration efforts reached into the coastal waters of various countries, the drilling techniques developed for use on land were adapted to fit an ocean environment. Thus, the initial efforts at undersea drilling for oil and gas began with the establishment of stable platforms from which to conduct drilling operations. The stability was typically achieved by constructing platforms supported on massive legs that extended down to the ocean floor. Once a well was brought into production, the oil or gas generally was not brought to the water's surface. Instead, the oil or gas from a producing well was conducted along the ocean floor by pipeline to onshore processing facilities.
Offshore exploration for oil and gas continues to move into ever deeper water farther and farther from land. With deeper water, the technique of constructing a stable platform by sinking supports to the ocean floor becomes more costly and less feasible. It also becomes more difficult and expensive to install and maintain pipelines to onshore facilities from oil or gas wells when the wells are drilled at depths of several thousand feet below the water's surface. Consequently, more recent developments in oil and gas exploration have concentrated on the construction of floating drilling platforms and floating preliminary processing facilities. The use of floating facilities has made it particularly necessary to devise suitable and acceptable methods for stabilizing such floating facilities and/or providing both drilling and production riser lines with the flexibility to accommodate the movements of a floating platform.
Although a floating drilling or production platform can be stabilized to some degree, the platform generally remains particularly susceptible to movements in response to wave action. Thus, for example, drilling that is done from a floating platform must accommodate both lateral and vertical motions of the platform. To accommodate such motions, drilling strings, riser lines and similar pipes or conduits which extend downwardly from the drilling platform to the ocean floor must either be provided with articulated joints or must inherently possess a degree of flexibility sufficient to prevent fracture or rupture of the conduits when the drilling platform moves or when waves or water currents act directly on the conduits. Typically, the pipe that is utilized in a drilling string, for example, is of a sufficiently small diameter and has sufficient strength to be flexible enough to avoid damage when an associated drilling platform is subjected to normal vertical or lateral movements. A riser line or marine conductor pipe, on the other hand, has a relatively large diameter and a consequently greater rigidity than a drilling string. Thus, such a large diameter pipe must include one or more couplings or joint assemblies that can be readily flexed and also withstand high internal and external fluid pressures.
One type of flexible joint used in riser pipes consists of a ball member having a precisely machined spherical surface and a socket member having a complementary spherical surface. The joint is flexed by sliding one of the spherical surfaces relative to the other. Resilient O-rings help seal the joint at the interface between the sliding surfaces. The flexural movement of such a ball joint is impaired, however, when the joint is subjected to high pressures. The joint is also subject to frictional wear and deterioration of both the sliding surfaces and the O-ring seals. The frictional wear requires frequent repair or replacement of the joint.
Another type of flexible joint for fluid conduits, such as marine riser pipes, utilizes annular flexible elements disposed between flanges secured to adjacent ends of different sections of conduit. The flexible elements generally comprise alternating layers of a nonextensible material and a resilient material, which are normally metal and an elastomer. The layers or laminations may be annular with flat surfaces, as in the pipe joint of Johnson U.S. Pat. No. 3,168,334, or annular with spherical surfaces, as in the flexible joint of Herbert et al U.S. Pat. No. 3,680,895. Laminated flexible elements permit the necessary flexural movement of a joint and can also function as seals. A joint incorporating a laminated element has no "moving" parts and is not subject to the frictional wear encountered with the ball-and-socket joints discussed above. Other flexible pipe joints utilizing laminated flexible elements are described and illustrated in Herbert et al U.S. Pat. Nos. 3,390,899, 3,734,546, and 3,853,337.
Although joints utilizing laminated flexible elements avoid the wear problem associated with conventional ball-and-socket joints, the laminated elements have a tendency to rupture and fail upon exposure to certain axial loads and to high pressure differentials. The elastomeric layers of laminated elements, while capable of carrying high compressive loads, can only withstand relatively low tension loads. Thus, when adjacent lengths of pipe are subjected to forces that tend to move the lengths axially so as to impose tension loads on the laminated element or elements in a joint connecting the pipe lengths, the laminated flexible elements are likely to fail. One approach to solving the problem of tension loads involves the use of tension bars to carry such tension loads, in preference to the laminated flexible elements. Pairs of laminated flexible elements may also be utilized in a joint such that at least one of the flexible elements is always loaded in compression, regardless of the direction of relative axial movement between adjacent lengths of pipe. The likelihood of rupture of a flexible element due to high pressure differentials exerted on it can be decreased by bonding adjacent laminations of the flexible element into an integral member. It has also been proposed, in copending, commonly owned patent application Ser. No. 821,448, filed Aug. 3, 1977, which is a continuation of abandoned patent application Ser. No. 621,433, filed Oct. 10, 1975, to configure a laminated flexible element so as to provide additional resistance to a high pressure differential acting primarily in one direction.